Method and apparatus for coating a tube interior



Nov. 7, 1961 R. H. HoBRocK METHOD AND APPARATUS FOR COATING A TUBE INTERIOR ATTORNEYS United States Patent Ofiiice 3,007,810 Patented Nov. 7, 1961 3,007,810 METHOD AND APPARATUS FOR COATING A TUBE INTERIOR Raymond H. Hobrock, Birmingham, Mich., assignor to Bundy Tubing Company, Detroit, Mich., a corporation of Michigan Filed Dec. 31, 1958, Ser. No. 784,168 9 Claims. (Cl. 117-96) This invention relates to a method and apparatus for providing a corrosion resistant coating of a low melting point alloy on the interiors of metal tubes such as might be used as compressed air lines in automotive pneumatic spring systems. These tubes have relatively small diameter which may range from about 4; to about For purposes of illustration tin and zinc alloys will be considered here.

It is desirable from the cost standpoint that the corrosion resistant coating be relatively thin and that no excess coating material be left on the tube interior. At the same time the coating must cover the entire tube interior to provide satisfactory corrosion resistance. It is both 'difiicult' and uneconomical to provide the coating by flooding the tube interior with a bath of molten coating material because excesses thereof tend to remain within the tube and thereby increase the cost of the tube. In this regard, the contemplated tubing is produced in large quantities and any unnecessary increase in cost, even by a fraction of a cent per foot is objectionable. Applying the coating by a spray method has not heretofore been feasible because the coating material tends to solidify before it can be distributed over the entire tube interior thus resulting in a faulty, discontinuous coating.

An object of this invention is to provide a simple improved inexpensive method and apparatus for applying a thin uniform complete coating of low melting point alloy on the interior surface of a tube. The method generally contemplates alloying tin or zinc with a relatively small percentage of another metal so that the resulting coating metal has a range of temperatures at which it has plastic properties. This alloy is sprayed onto the tube interior and can be distributed in a thin continuous coating on the interior wall of the tube before it cools below the plastic range of temperatures and solidifies. In general, the apparatus includes a hollow mandrel with a nozzle adjacent its end over which the tube is advanced and through which the molten coating alloy is sprayed onto the tube interior. Skewed rolls spin the tube as it is moved longitudinally to centrifugally spread the alloy over the tube interior and the mandrel includes an enlarged wiper portion adjacent this nozzle to assist in spreading the alloy.

7 The drawing illustrates diagrammatically apparatus which may be used for practicing the invention.

The apparatus illustrated in the drawings includes a vessel 1 heated by suitable means such as electric heater coils 2 .to maintain in molten condition a body 3 of coating metal. A pump 4 operated by motor 5 is provided for forcing the molten coating metal through a pipe 6 containing a valve 7 and having an elongate hori zontal branch 8 adjacent the end of which is a nozzle comprised of a number of circumferentially arranged outlet ports 9. Pipe 8 has an enlargement or bulbous portion 12 at its end and adjacent the nozzle portion. Pipe 8 with its nozzle ports 9 and enlargement 12 may be termed for convenience a hollow mandrel M. Rolls 13 and 14 are provided for engaging enlargement 12 and thereby supporting the nozzle end of pipe 8. These rolls are urged toward enlargement 12 by springs 15 and 16 which yield to permit the end of tube T to be inserted between the rolls and enlargement 12. Rolls 13 and 14 are skewed for a purpose which will become apparent.

Means are provided for advancing the tube T longitudinally over mandrel M, spinning the tube about its longitudinal axis and heating the tube. These means comprise a number of sets of skewed rolls 20-29 for engaging the exterior of tube T. Some-or all of the sets of rolls on both sides of mandrel enlargement 12 may be driven. A guide 30 may be provided to facilitate feeding tube T between skewed rolls 20-23. A transformer 32 has a primary 33 and a secondary 34 one side of which is connected to rolls 14 and 25 and the other side of which is connected to roll 27 and to pipe 6-8 as illustrated. Rolls 14, 25, and 27 and pipe 6-8 are electric conductors.

Generally speaking, tube T is advanced through guide 30, rolls 20-23, rolls 13, 14 and rolls 2433 in a direction toward the left as the drawing is viewed until its end is adjacent pipe 6. Suitable electric circuits are provided for heating the portion of the tube to the left of nozzle 9 and the tube is then retracted toward the right with its interior being sprayed with a coating alloy. The tube is cooled by a spray of coolant 36issuing from a pipe 37 having a valve 38. The electric circuits for heating the tube and for operating valves 7 and 38 and the drive for the skewed rolls may be controlled by switches such as conventional magnetic proximity switches 39-41, a timer 42 and switches 43 and 44 actuated respectively by relays 45 and 46. Detailed operation is described below.

The coating metal has as its principal constituent either tin or zinc. Each of these metals in pure form has a relatively sharp freezing point which would make it very difiicult to spread it over the tube interior in a complete continuous coating before it solidifies. Consequently, other metals are added to the tin or zinc so that the resulting alloy will have plastic properties in a range of temperatures between which the alloy is in its liquid state and solid state. This enables the alloy to be evenly applied in a complete continuous coating around'the tube interior even though there may be considerable temperature variation at the portions of the tube being coated.

A number of metals can be added to tin to provide this plastic temperature range. Three examples follow:

(1) Tin about 99%, aluminum about 1%: This alloy has plastic properties in the range from about 232 C. to about 300 C. a

(2) Tin about to 99%, cadmium about 1% to about 10%: This alloy has plastic properties in a range from about 177 C. to about 232 C.

(3) Tin about %,'antimony about 5%: This alloy has plastic properties in a range from about 232 C. to about 247 C. Where zinc is the principal constituent the alloy may comprise about 95% zinc and about 5% tin and the resulting alloy will have plastic properties in a range between about 200 C. and 400 C.

Thus when any of these alloys is used, the coating may be applied to the tube interior at a temperature which may lie within a relatively large range of temperatures and the heat conditions need not be controlled with great precision. A tin coating may be used where the coating itself is adequate to provide the desired protection against corrosion. The zinc alloy may be used in instances where the added protection of galvanic action is desirable.

As to operation of the apparatus illustrated, it may be assumed that initially no tube T has been introduced between the various sets of rolls. Spring pressed rolls 13 and 14 engage mandrel enlargement 12 for supporting pipe 8 and an electric circuit is established between one side of secondary 34, pipe 6-8, enlargement 12, roll 14 and the other side of the secondary. Pipe 6-8 is thus heated to maintain coating metal therein in molten condition. Pump 4 is in operation, valves 7 and 38 are closed, relays 45 and 46 are de-energized and switches 43 and 44 are open. One or more rolls 20-49 on each side of mandrel enlargement 12 are being driven in a direction for advancing a tube T toward the left as the drawing is viewed.

Before a tube T is introduced into the apparatus, its interior may have been treated with a proper flux or may otherwise have been prepared for adherence of the alloy coating. When the end of tube T is introduced through guide. 30 and between rolls 20-23 these rolls spin the tube and advance it longitudinally toward the left. Rolls 16 and 14 spread apart to allow the tube to pass over mandrel enlargement 12 and the tube continues to advance through rolls 24-29 until the tube end approaches magnetic proximity switch 41. Switch 41 actuates circuits for stopping the drive for rolls 2029, releasing timer 42 and energizing relays 45 and 46 for closing switches 43 and 44. The tube remains at rest for a period determined by the timing-out period of timer 42 and during this period, current passes through :and heats the portion of the tube between rolls 26, 27, and rolls 16, 14. The heating period is sufficient to heat the tube to a temperature lying generally within the range of temperatures at which the metal alloy being used has plastic properties. During this period the heatingcircuit for pipe 6-8 is maintained since the tube wall is an electric conductor and passes current from, roll 14to mandrel enlargement 12. Consequently, thealloy within pipe 8 remains molten.

When timer 42 times out it actuates suitable circuits for opening valve 7, starting the drive for rolls 20--29 so that they retract tube T toward the right, and for opening coolant spray valve 38. Molten coating alloy is sprayed from nozzle ports 9 onto the tube interior. As the tube is moved longitudinally to the right it is spun about its longitudinal'axis by the action of the skewed rolls and the coating metal is spread uniformly around the tube interior by centrifugal force. Enlargement 12 may provide a wiper for smoothing the coating of metal on the tube interior. There may be some cooling of the metal between nozzle ports 9 and enlargement 12 but so long .as the metal does not cool below the range of temperatures at which it has plastic properties this cooling will have no harmful effect. As illustrated by the above examples, the. temperature of the alloy may vary through a substantial range without danger of the coating metal solidifying before it has been spread uniformly in a continuous coating around the tube interior. Thus no precise temperature controlling apparatus need be provided. It will be noted that as the tube is advanced toward nozzle 9 it is heated by current passing therethrough from rolls '27--25 and 14.

After the tube with its interior metal coating passes mandrel enlargement 12 it is subjected to cooling which is preferably relatively sudden in nature to chill the coating metal and solidify it. For this purpose the tube is preferably sprayed 'as at 36 with coolant immediately downstream from mandrel enlargement '12. When the end of tube T' approaches magnetic proximity switch 40, adjacent nozzle 9, this switch actu-ates a circuit for closing valve 7 to cut off the supply of molten metal to nozzles 9. As the end of the tube then-is retracted past magnetic proximity switch 39 this switch energizes suitable circuits for closing spray valve 38, reversing the drive for skewed rolls 2029 and tie-energizes relays 45 and 46 for opening switches 43- and 44. Alt the same time, timer 42 may be reset and other circuits conditioned for repeating the cycle when another tube is introduced into the apparatus.

I claim:

1. The method of coating the interior of a tube with a corrosion-resistant metal which comprises, maintaining in molten condition a body of coating metal in the form of an alloy whose constituents are predetermined so that said coating metal has plastic properties in a range of temperatures between the temperatures at which it is in solid and liquid states, subjecting said tube to heat in a zone to heat said tube at least to a temperature lying within said range 4 of temperatures, spinning said tube about its longitudinal axis and at the same time advancing it longitudinally so that it passes out of said heating zone and into a coating zone generally adjacent the location where it leaves said heating zone and then passes from said coating zone directly into a cooling zone, applying said molten coating metal to the tube interior at said coating zone while said tube is spinning and advancing longitudinally, controlling the temperature of the tube so that between said coating zone and cooling zone said coating metal remains within said range of temperatures and spreads centrifugally substantially uniformly over the tube interior, and at said cooling zone, while said tube is advancing and spinning, cooling said coating metal below said range of -tempera tures to solidify it while it is substantially uniformly distributed over the tube interior.

2. The method defined in claim 1 wherein said metal alloy contains about 95% zinc and about 5% tin.

3, The method defined in claim 1 wherein said body of metal alloy includes tin alloyed with a metal selected from the group consisting of aluminum, cadmium, antimony and zinc.

4. The method defined in claim 3 wherein said body of metal alloy contains approximately 99% tin and 1% aluminum.

5. The method defined in claim 3 wherein said metal alloy contains approximately to. 99% tin and ap proximately 1% to 10% cadmium.

6. The method defined in claim 3 wherein said metal alloy contains approximately tin and approximately 5% antimony.

7. The method of coating the interior of a tube with a corrosion-resistant metal which comprises, providing a body of coating metal in the form of an alloy whose constituents are predetermined so that said coating metal has plastic properties in a range of temperatures betweenthe temperatures at which it is in solid and liquid states, maintaining said body of metal in molten condition, furnishing said molten metal under pressure to a spray nozzle, providing a heating zone disposed at one side of said nozzle and terminating generally adjacent said nozzle, providing a cooling zone disposed at a side of said nozzle opposite from said heating zone, placing said tube in said heating zone in longitudinal alignment with said nozzle, subjecting said tube to heat in said heating zone to heat said tube at least to a temperature lying within said range of temperatures, spinning said tube about its longitudinal axis and at the same time advancing it longitudinally out of said heating zone and over said nozzle, spraying said molten metal onto the interior surface of the tube as it spins and advances past said nozzle, controlling the temperature of the tube so that at a zone immediately downstream of said nozzle it remains within said range of temperatures and spreads centrifugally substantially uniformly over the tube interior, passing the tube from said zone downstream of said nozzle directly into said cooling zone, and at said cooling zone, while said tube is advancing and spinning, cooling said metal below said range of temperatures to solidify it while it is substantially uniformly distributed over the tube interior..

8. Apparatus for coating the interior of tubing comprising, a reservoir for molten coating metal, a hollow mandrel adapted to extend into tubing with which said apparatus is adapted to be used, said mandrel having an.

outlet nozzle disposed in a coating zone, heating means disposed in a heating zone adjacent one side of said coating zone, coo-ling means disposed in a cooling zone on the opposite side of said coating zone from said heating zone, tube-moving means operable to advance tubing axially through said cooling zone, through said coating zone, and over said mandrel and to said heating zone, means operable to inactuate said tube-moving means to facilitate heating of the tube in said heating zone, means operable to reverse said tube-moving means, the reversed moving means being operable to spin the heated tube on its longitudinal axis and retract the spinning tube axially from said heating zone, over said mandrel, and through said coating zone and then directly into said cooling zone, means operable to introduce molten coating metal from said reservoir into said mandrel so that metal is sprayed into the tube interior as the tube spins and retracts past said nozzle whereby to progressively apply and centrifugally distribute coating metal on the tube interior in said coating zone, said cooling means being operable to chill the spinning and longitudinally moving tube in said cooling zone to solidify the coating metal in its uniformly distributed condition on the tube interior.

9. The combination defined in claim 8 wherein said mandrel is provided with an enlarged portion disposed with n said coating zone downstream of said nozzle, 15

5 centrifugal distribution thereof.

References Cited in the file of this patent UNITED ST PATENTS 2,162,829 Stuart et al. June 20, 1939 2,676,895 Russel Apr. 27, 1954 2,822,291 Hahn Feb. 4, 1958 FOREIGN PATENTS 415,732 Great Britain Aug. 27, 1934 491,868 Great Britain Sept. 12, 1938 

1. THE METOD OF COATING THE INTERIOR OF A TUBE WITH A CORROSION-RESISTANT METAL WHICH COMPRISES, MAINTAINING IN MOLTEN CONDITION A BODY OF COATING METAL IN THE FORM OF AN ALLOY WHOSE CONSTITUENTS ARE PREDETERMINED SO THAT SAID COATING METAL HAS PLASTIC PROPERTIES IN A RANGE OF TEMPERATURES BETWEEN THE TEMPERATURES AT WHICH IT IS IN SOLID AND LIQUID STATES, SUBJECTING SAID TUBE TO HEAT IN A ZONE TO HEAT SAID TUBE AT LEAST TO A TEMPERATURE LYING WITHIN SAID RANGE OF TEMPERATURES, SPINNING SAID TUBE ABOUT ITS LONGITUDINAL AXIS AND AT THE SAME TIME ADVANCING IT LONGITUDINALLY SO THAT IT PASSES OUT OF SAID HEATING ZONE AND INTO A COATING ZONE GENERALLY ADJACENT THE LOCATION WHERE IT LEAVES SAID HEATING ZONE AND THEN PASSES FROM SAID COATING ZONE DIRECTLY INTO A COOLING ZONE, APPLYING SAID MOLTEN COATING METAL TO THE TUBE INTERIOR AT SAID COATING ZONE WHILE SAID TUBE IS SPINNING AND ADVANCING LONGITUDINALLY, CONTROLLING THE TEMPERATURE OF THE TUBE SO THAT BETWEEN SAID COATING ZONE AND COOLING ZONE SAID COATING METAL REMAINS WITHIN SAID RANGE OF TEMPERATURES AND SPREADS CENTRIFUGALLY SUBSTANTIALLY UNIFORMLY OVER THE TUBE INTERIOR, AND AT SAID COOLING ZONE, WHILE SAID TUBE IS ADVANCING AND SPINNING, COOLING SAID COATING METAL BELOW SAID RANGE OF TEMPERATURES TO SOLIDIFY IT WHILE IT IS SUBSTANTIALLY UNIFORMLY DISTRIBUTED OVER THE TUBE INTERIOR. 